Any discussion of the background art throughout the specification should in no way be considered as an admission that such background art is prior art, nor that such background art is widely known or forms part of the common general knowledge in the field.
Carbon nanotubes, an allotrope of carbon with a cylindrical nanostructure, have attracted a considerable amount of interest recently due to their unique properties. Their use is highly attractive across many fields of technology, including electronics, optics and other fields of materials science, as well as potential uses in construction methods. Nanotubes are characterised by their extremely large length-to-diameter ratio of many million times to 1 and exhibit extraordinary strength and unique electrical properties, as well as high efficiency as thermal conductors.
There are various conventional methods of producing carbon nanotubes: electric arc discharge, catalytic hydrocarbon decomposition, laser ablation, high pressure carbon monoxide (HiPco), thermal chemical deposition, plasma chemical deposition and others.
Nanotubes are formed as graphite sheets (also known as graphene), in which atoms of carbon are arranged hexagonally in a sheet a single atom thick, and wrapped into a seamless cylinder. A cylinder formed from a single graphite sheet is a known as single-walled nanotubes, and typically has a diameter between about one nanometer to a few tens of nanometers (about 30 to 50 nm) and a length that may be many orders of magnitude longer. Multi-walled nanotubes may also be formed from concentric cylinders of graphite sheet, itself comprising several graphite sheets, which may be between two and about 50 walls, typically between about 2 to 10. Such multi-walled nanotubes may have a diameter between about a few (about 2 to 5 nm) nanometers to a few tens of nanometers (about 30 to 50 nm). Single-walled carbon tubes are primarily produced using an arc discharge method utilizing carbon electrodes in an environment containing a metallic catalyst, or where the anode electrode used for producing the arc discharge comprises a metallic catalyst substance. The main limitations of the arc discharge method, however is low nanotube yield, typically no more than about 25% to 30% by weight of total carbon mass, the relatively small size of the nanotubes produced (typically having lengths of up to about 50 to 1000 nanometers, difficulties associated with separating out the nanotubes in a pure form, and difficulties with varying diameter and length dimensions of the nanotubes formed by this process.
Catalytic methods of carbon nanotubes production can overcome many of the difficulties with arc discharges methods, and by varying the conditions of the catalytic carbon nanotube synthesis, formation of undesirable amorphous carbon may be drastically reduced. Varying catalyst parameters and conditions of decomposition of hydrocarbon feedstock, carbon nanotubes diameter and length can be changed together with their high yield (on a weighted basis) together with low concentration, or the lack of amorphous carbon. Separating the formed nanotubes is also easier using catalytic synthesis methods, where carbon material may be separated from metal and oxides using ultrasound or one of a variety of different chemical treatment methods, thus permitting pure nanotubes with open ends to be readily obtained. Also, using catalytic synthesis methods, it is possible to obtain straight, inclined and twisted nanotubes of fullerene diameters, representing both theoretical and practical interest.
However, despite these advantages, present catalytic synthesis methods of carbon nanotubes synthesis are high cost, which is driven by large volume of catalyst usage and low yield of carbon nanotubes per unit of mass of catalyst. The are methods for producing carbon nanotubes which output a yield per unit mass of catalyst of greater than 100, however, the main obstacle affecting the yield using conventional techniques is the high cost of obtaining the catalyst itself. In current processing methods for producing a catalyst, the preparation of the catalyst and subsequent processes for the synthesis of nanotubes are separated into different technological processes running on different process equipment.
The main disadvantage of current nanotube production processes is that they are multistage processes. Typically, the process of manufacturing the catalyst goes through several stages. The catalyst is typically prepared in the form of either agglomerates or in a powder form on a substrate. In the case of catalysts in agglomerate form, problems are encountered with separation of entangled nanotubes and fibers synthesized on the catalyst. In the case of a catalyst formed on a substrate, problems are encountered in separation of the synthesized nanotubes from the substrate.
Therefore, a need exists for an improved process for catalyst preparation and synthesis of nanotubes. There is also a need for a low cost, yet high yield method for rapid synthesis of large quantities of high quality pure carbon nanotubes with uniform dimensionality to meet the demands of the many varied technological applications for these nanotubes.
It is therefore an object of the present invention to address the disadvantages of the prior art, or at least to provide a useful alternative to existing synthesis methods for carbon nanotubes, and/or systems and apparatus for their production in large quantities.